Soybean seedling diseases and seed treatment in Spring

As the use of no-till farming practices increases, the number of questions about potential problems of seedling disease in early planted soybean or in no-till soybean fields increases.

Information on the occurrence of seedling disease in no-till is limited. However, a two-year study by researchers at the ISU soybean disease laboratory indicated that less than 1 percent of Iowa soybean fields had significant stand establishment problems. Last year, we conducted seed treatment experiments in six selected fields at three locations using different fungicides. Five out of six fields showed increased stand counts. One field, known to have seedling disease, showed significant increase in stand establishment in areas where treated seeds were used. Our results indicated that the benefits of fungicide treatments are consistent if disease risk is high.

How can you assess the risk of seedling diseases in a soybean field? Consider the following situations, under which risk of seedling disease may be high:

You want to plant early in fields that had seedling diseases or a history of replanting in the last few years. A replanting history (unless it was caused by hail) or previous seedling disease experience indicates the presence of pathogenic fungi in your fields. If you plant early, when soil is cold and wet, the chance of infection by fungi increases.

You use low quality seeds and want to plant early. ISU research showed consistent benefits of fungicide treatment if low quality soybean seeds were planted at a soil temperature of 50F (Table 1). Planting low quality seeds in cooler soil increases the risk of disease. The ISU Seed Science Centers 1994 seed testing results showed that the cold germination rate of soybeans from some lots was significantly lower than normal. So, if you want to plant early, ask your seed company about the cold germination rate of their seeds. If the cold germination rate is low, seed treatment could be beneficial. Table 1 provides information for your use.(not available on-line, see printed copy)

Neither early planting nor the use of no-till alone indicates a decision to use seed treatment. Do not consider seed treatment unless there is a significant risk of seedling disease.

You can get commercially treated soybean seeds, or you can treat your own seeds at planting. Because seedling disease is not a wide-spread problem in Iowa, only a very small portion of commercial soybean seeds are pre-treated. For on-farm treatment , hopper box treatment, mixing fungicide with the seeds in the hopper boxes at planting, is most commonly used. To be effective, the chemical must be fully mixed with the seeds in a hopper box.

ISU Research showed that Rhizoctonia, Pythium, and Phytophthora are the major fungi, causing up to 85 percent of seedling disease problems in Iowa (see the ICM newsletter 1994:190). About 10 percent of seedling diseases are caused by Fusarium. Choose chemicals targeted to these dominant fungi; some contain compounds that control several fungi.